[T-Bone Texan]

you can put a .38 round into a .357 gun, but you should not attempt the reverse.

[Fightin Whitey]

but you should not attempt the reverse

Good point.

[Manly Warrior]

That’s because you cannot fully chamber a 357 Mag in a 38 spl chamber and close the cylinder- the extra long cartridge was made to prevent such a mistake.

The 38 spl, originally a black powder round, operates at around 16000 psi, the 357, designed in the early 30s to penetrate car doors etc, used smokeless powder and operates at ~35000 psi. Modern metallurgy and design changes by SW enabled such a performance jump.